A quantum dot is a nanocrystalline material (also referred to herein as a nanoparticle) having a centre region typically of size 1-100 nm formed of a semiconductor having a first bandgap, the centre region being surrounded by a second region having a second bandgap, wherein the second bandgap is larger than the first. The second region may be, for example, a vacuum, or a second semiconductor. An example of a quantum dot uses cadmium selenide (CdSe) as the centre region and zinc sulphide (ZnS) as the surrounding second region, with ZnS having a larger bandgap than CdSe.
Quantum dots have tunable electronic, optical and magnetic properties depending on the diameter of the nanocrystals from which they are formed[1].
The Stark effect is the shifting and splitting of the spectral lines of atoms and molecules due to the application of an electric field. The amount of splitting or shifting may respectively be referred to as the Stark splitting or Stark shift. Since the frequency of light absorbed (or in some cases emitted) by the atoms or molecules is determined by the frequencies of the spectral lines, the frequency of the absorbed (or emitted) light can be changed, via the Stark effect, by the application of an electric field.
As those skilled in the art will readily appreciate, the term “light” as used herein should be interpreted broadly, to encompass not only visible light but also other wavelengths of electromagnetic radiation outside the visible region. Similarly, the term “optical” as used herein should be interpreted broadly, to encompass not only systems which operate using visible light but also those which operate outside the visible region of the electromagnetic spectrum.
In a semiconductor heterostructure, where a small bandgap material is sandwiched between two layers of a larger bandgap material, the Stark effect can be enhanced by bound excitons. This is known as the quantum-confined Stark effect. The electron and hole which form the exciton are pulled in opposite directions by the applied electric field, but they remain confined in the smaller bandgap material, so the exciton is not merely pulled apart by the field. In practice, the quantum-confined Stark effect has been used for semiconductor-based optical modulators, particularly for optical fiber communications.
However, to date, control of the fundamental absorption edge of a quantum dot with an applied electric field, through the Stark effect [2], has been limited by the electrical breakdown of the material surrounding the dot. This has limited the range of wavelengths over which the fundamental absorption edge of the quantum dot may be tuned.
There is therefore a desire to be able to apply much larger fields to quantum dots, in order to be able to vary the fundamental absorption edge of the quantum dot over a greater range of wavelengths, and thus obtain improved tunable optical properties.